The permanent-magnet flowmeter has proved to be very satisfactory for measuring flow of electrically conducting fluids in extreme environments such as those experienced by liquid sodium in a liquid-metal nuclear reactor. However, the response fo such flowmeters is a function of the magnetic field strength of the permanent magnet in the flowmeter, and any change affecting this magnet is accompanied by a corresponding change in the response characteristics of the flowmeter. The permanent magnet is in an environment of high temperature and frequently of relatively high nuclear radiation and therefore changes in the magnetic field strength are the rule rather than the exception. The same conditions that produce such changes also make it frequently difficult to calibrate and recalibrate such flowmeters. What is needed is a system that allows use of the permanent magnet flowmeter while allowing it to be readily recalibrated against an absolute standard as often as is necessary to preserve reliability of its reading.
Many other methods of measuring the flow of liquid metals are in use or under consideration, including eddy-current devices, turbines, orifice methods for converting pressure drops to velocities and hence to flow rates, and thermal transit-time devices. All but the thermal transit-time devices are extensions to liquid-metal uses of devices well known in other applications that measure fluid flow. The thermal transit time method determines the correlation between a-c electrical signals from two thermocouples separated from each other along the axis of a flow channel. The thermocouples are selected for fast time response. However, the term "fast time response" is relative and the time required to achieve sufficient measurements to determine statistical correlation to a satisfactory degree of probability is still of the order of minutes. This requires that the velocity of flow in a channel remain unchanged over the period necessary to gather data for correlation and it renders the thermocouple method unsatisfactory for providing loss-of-flow or change-of-flow information in situations in which a rapid response is desired.
It is an object of the present invention to provide a better measure of the flow of a conducting fluid.
It is a further object of the present invention to provide a method of calibrating a permanent-magnet sodium flowmeter.
It is a further object of the present invention to provide a method and means of measuring the flow of an electrically conducting fluid that is absolute, fast, and reliable.
It is a further object of the present invention to provide a better method of monitoring the flow of a conducting liquid metal in a nuclear reactor.
Other objects will become apparent in the course of a detailed description of the invention.